NF-kappaB1 p105 negatively regulates TPL-2 MEK kinase activity

Activation of the oncogenic potential of the MEK kinase TPL-2 (Cot) requires deletion of its C terminus. This mutation also weakens the interaction of TPL-2 with NF-kappaB1 p105 in vitro, although it is unclear whether this is important for the activation of TPL-2 oncogenicity. It is demonstrated here that TPL-2 stability in vivo relies on its high-affinity, stoichiometric association with NF-kappaB1 p105. Formation of this complex occurs as a result of two distinct interactions. The TPL-2 C terminus binds to a region encompassing residues 497 to 534 of p105, whereas the TPL-2 kinase domain interacts with the p105 death domain. Binding to the p105 death domain inhibits TPL-2 MEK kinase activity in vitro, and this inhibition is significantly augmented by concomitant interaction of the TPL-2 C terminus with p105. In cotransfected cells, both interactions are required for inhibition of TPL-2 MEK kinase activity and, consequently, the catalytic activity of a C-terminally truncated oncogenic mutant of TPL-2 is not affected by p105. Thus, in addition to its role as a precursor for p50 and cytoplasmic inhibitor of NF-kappaB, p105 is a negative regulator of TPL-2. Insensitivity of C-terminally truncated TPL-2 to this regulatory mechanism is likely to contribute to its ability to transform cells.     

Direct phosphorylation of NF-kappaB1 p105 by the IkappaB kinase complex on serine 927 is essential for signal-induced p105 proteolysis

The p105 precursor protein of NF-kappaB1 acts as an NF-kappaB inhibitory protein, retaining associated Rel subunits in the cytoplasm of unstimulated cells. Tumor necrosis factor alpha (TNFalpha) and interleukin-1alpha (IL-1alpha) stimulate p105 degradation, releasing associated Rel subunits to translocate into the nucleus. By using knockout embryonic fibroblasts, it was first established that the IkappaB kinase (IKK) complex is essential for these pro-inflammatory cytokines to trigger efficiently p105 degradation. The p105 PEST domain contains a motif (Asp-Ser(927)-Gly-Val-Glu-Thr), related to the IKK target sequence in IkappaBalpha, which is conserved between human, mouse, rat, and chicken p105. Analysis of a panel of human p105 mutants in which serine/threonine residues within and adjacent to this motif were individually changed to alanine established that only serine 927 is essential for p105 proteolysis triggered by IKK2 overexpression. This residue is also required for TNFalpha and IL-1alpha to stimulate p105 degradation. By using a specific anti-phosphopeptide antibody, it was confirmed that IKK2 overexpression induces serine 927 phosphorylation of co-transfected p105 and that endogenous p105 is also rapidly phosphorylated on this residue after TNFalpha or IL-1alpha stimulation. In vitro kinase assays with purified proteins demonstrated that both IKK1 and IKK2 can directly phosphorylate p105 on serine 927. Together these experiments indicate that the IKK complex regulates the signal-induced proteolysis of NF-kappaB1 p105 by direct phosphorylation of serine 927 in its PEST domain.     

Asbestos-mediated CREB phosphorylation is regulated by protein kinase A and extracellular signal-regulated kinases 1/2

Asbestos is a ubiquitous, naturally occurring fiber that has been linked to the development of malignant and fibrotic lung diseases. Asbestos exposure leads to apoptosis, followed by compensatory proliferation, yet many of the signaling cascades coupled to these outcomes are unclear. Because CREs (Ca(2+)/cAMP-response elements) are found in the promoters of many genes important for regulation of proliferation and apoptosis, CREB (CRE binding protein) is likely to play an important role in the development of asbestos-mediated lung injury. To explore this possibility, we tested the hypotheses that asbestos exposure leads to CREB phosphorylation in lung epithelial cells and that protein kinase A (PKA) and extracellular signal-regulated kinases 1/2 (ERK1/2) are central regulators of the CREB pathway. Persistent CREB phosphorylation was observed in lung sections from mice following inhalation of crocidolite asbestos. Exposure of C10 lung epithelial cells to crocidolite asbestos led to rapid CREB phosphorylation and apoptosis that was decreased by the inhibition of PKA or ERK1/2 using the specific inhibitors H89 and U0126, respectively. Furthermore, crocidolite asbestos selectively induced a sustained increase in MAP kinase phosphatase-1 mRNA and protein. Silencing CREB protein dramatically reduced asbestos-mediated ERK1/2 phosphorylation, yet significantly increased the number of cells undergoing asbestos-induced apoptosis. These data reveal a novel and selective role for CREB in asbestos-mediated signaling through pathways regulated by PKA and ERK1/2, further providing evidence that CREB is an important regulator of apoptosis in asbestos-induced responses of lung epithelial cells.     

Integrins regulate microtubule nucleating activity of centrosome through mitogen-activated protein kinase/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase (MEK/ERK) signaling

Microtubule nucleation is an essential step in the formation of the microtubule cytoskeleton. We recently showed that androgen and Src promote microtubule nucleation and γ-tubulin accumulation at the centrosome. Here, we explore the mechanisms by which androgen and Src regulate these processes and ask whether integrins play a role. We perturb integrin function by a tyrosine-to-alanine substitution in membrane-proximal NPIY motif in the integrin β1 tail and show that this mutant substantially decreases microtubule nucleation and γ-tubulin accumulation at the centrosome. Because androgen stimulation promotes the interaction of the androgen receptor with Src, resulting in PI3K/AKT and MEK/ERK signaling, we asked whether these pathways are inhibited by the mutant integrin and whether they regulate microtubule nucleation. Our results indicate that the formation of the androgen receptor-Src complex and the activation of downstream pathways are significantly suppressed when cells are adhered by the mutant integrin. Inhibitor studies indicate that microtubule nucleation requires MEK/ERK but not PI3K/AKT signaling. Importantly, the expression of activated RAF-1 is sufficient to rescue microtubule nucleation inhibited by the mutant integrin by promoting the centrosomal accumulation of γ-tubulin. Our data define a novel paradigm of integrin signaling, where integrins regulate microtubule nucleation by promoting the formation of androgen receptor-Src signaling complexes to activate the MEK/ERK signaling pathway.     

Effects of resistance exercise intensity on extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase activation in men

Extracellular signal-regulated kinase (ERK) 1/2 signaling has been shown to be increased after heavy resistance exercise and suggested to play a role in the hypertrophic adaptations that are known to occur with training. However, the role that ERK1/2 may play in response to lower intensities of resistance exercise is unknown. Therefore, the purpose of this study was to determine the effects of resistance exercise intensity on ERK1/2 activity in human skeletal muscle. Twelve recreationally active men completed separate bouts of single-legged resistance exercise with 8-10 repetitions (reps) at 80-85% 1 repetition maximum (1RM) (85%) and 18-20 reps at 60-65% 1RM (65%) in a randomized crossover fashion. For both resistance exercise sessions, vastus lateralis biopsies and blood draws were taken immediately before exercise (PRE) and at 30 minutes (30MPST), 2 hours (2HRPST), and 6 hours (6HRPST) post exercise, with an additional blood draw occurring immediately after exercise (POST). The phosphorylated levels of pIGF-1R, pMEK1, pERK1/2, and activated Elk-1 were assessed by phosphoELISA, and serum insulin-like growth factor 1 (IGF-1) was assessed via enzyme-linked immunosorbent assay. Statistical analyses used a 2 × 4 (muscle responses) and 2 × 5 (serum responses) multivariate analysis of variance on delta values from baseline (p < 0.05). Both exercise intensities significantly increased the activity of insulin-like growth factor 1 receptor (IGF-1R), mitogen-activated protein kinase 1, ERK1/2, and Elk-1, with peak activity occurring at 2HRPST (p < 0.001). However, 65% resulted in a preferential increase in IGF-1R and Elk-1 activation when compared with 85% (p < 0.05). No differences were observed for serum IGF-1 levels regardless of intensity and time. These findings demonstrate that resistance exercise upregulates ERK1/2 signaling in a manner that does not appear to be preferentially dependent on exercise intensity.     

The mitogen-activated protein kinase extracellular signal-regulated kinase 1/2 pathway is involved in formyl-methionyl-leucyl-phenylalanine-induced p47phox phosphorylation in human neutrophils

Phosphorylation of p47 phagocyte oxidase, (p47(phox)), one of the NADPH oxidase components, is essential for the activation of this enzyme and for superoxide production. p47(phox) is phosphorylated on multiple serine residues, but the kinases involved in this process in vivo remain to be characterized. We examined the role of extracellular signal-regulated kinase (ERK1/2) and p38 mitogen-activated protein kinase in p47(phox) phosphorylation. Inhibition of ERK1/2 activation by PD98059, a specific inhibitor of ERK kinase 1/2, inhibited the fMLP-induced phosphorylation of p47(phox). However, PD98059 weakly affected PMA-induced p47(phox) phosphorylation, even though ERK1/2 activation was abrogated. This effect was confirmed using U0126, a second ERK kinase inhibitor. Unlike PD98059 and U0126, the p38 mitogen-activated protein kinase inhibitor SB203580 did not inhibit the phosphorylation of p47(phox) induced either by fMLP or by PMA. Two-dimensional phosphopeptide mapping analysis showed that, in fMLP-induced p47(phox) phosphorylation, PD98059 affected the phosphorylation of all the major phosphopeptides, suggesting that ERK1/2 may regulate p47(phox) phosphorylation either directly or indirectly via other kinases. In PMA-induced p47(phox) phosphorylation, GF109203X, a protein kinase C inhibitor, strongly inhibits p47(phox) phosphorylation. However, in fMLP-induced p47(phox) phosphorylation, PD98059 and GF109203X partially inhibited the phosphorylation of p47(phox) when tested alone, and exerted additive inhibitory effects on p47(phox) phosphorylation when tested together. These results show for the first time that the ERK1/2 pathway participates in the phosphorylation of p47(phox). Furthermore, they strongly suggest that p47(phox) is targeted by several kinase cascades in intact neutrophils activated by fMLP and is therefore a converging point for ERK1/2 and protein kinase C.     

Mechanistic basis for catalytic activation of mitogen-activated protein kinase phosphatase 3 by extracellular signal-regulated kinase

The dual specificity mitogen-activated protein kinase phosphatase MKP3 has been shown to down-regulate mitogenic signaling through dephosphorylation of extracellular signal-regulated kinase (ERK). Camps et al. (Camps, M., Nichols, A., Gillieron, C., Antonsson, B., Muda, M., Chabert, C., Boschert, U., and Arkinstall, S. (1998) Science 280, 1262-1265) had demonstrated that ERK binding to the noncatalytic amino-terminal domain of MKP3 can dramatically activate the phosphatase catalytic domain. The physical basis for this activation has not been established. Here, we provide detailed biochemical evidence that ERK activates MKP3 through the stabilization of the active phosphatase conformation, inducing closure of the catalytic "general acid" loop. In the closed conformation, this loop structure can participate efficiently in general acid/base catalysis, substrate binding, and transition-state stabilization. The pH activity profiles of ERK-activated MKP3 clearly indicated the involvement of general acid catalysis, a hallmark of protein-tyrosine phosphatase catalysis. In contrast, unactivated MKP3 did not display this enzymatic group as critical for the low activity form of the enzyme. Using a combination of Br?nsted analyses, pre-steady-state and steady-state kinetics, we have isolated all catalytic steps in the reaction and have quantified the specific rate enhancement. Through protonation of the leaving group and transition-state stabilization, activated MKP3 catalyzes formation of the phosphoenzyme intermediate approximately 100-fold faster than unactivated enzyme. In addition, ERK-activated MKP3 catalyzes intermediate hydrolysis 5-6-fold more efficiently and binds ligands up to 19-fold more tightly. Consistent with ERK stabilizing the active conformation of MKP3, the chemical chaperone dimethyl sulfoxide was able to mimic this activation. A general protein-tyrosine phosphatase regulatory mechanism involving the flexible general acid loop is discussed.     

Farrerol maintains the contractile phenotype of VSMCs via inactivating the extracellular signal-regulated protein kinase 1/2 and p38 mitogen-activated protein kinase signaling

Molecular and Cellular Biochemistry - Farrerol, a dihydroflavone isolated from Rhododendron dauricum L., can inhibit vascular smooth muscle cell (VSMC) proliferation and exert a protective effect...     

Pharmacological and genetic evaluation of proposed roles of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK), extracellular signal-regulated kinase (ERK), and p90(RSK) in the control of mTORC1 protein signaling by phorbol esters

The mammalian target of rapamycin complex 1 (mTORC1) links the control of mRNA translation, cell growth, and metabolism to diverse stimuli. Inappropriate activation of mTORC1 can lead to cancer. Phorbol esters are naturally occurring products that act as potent tumor promoters. They activate isoforms of protein kinase C (PKCs) and stimulate the oncogenic MEK/ERK signaling cascade. They also activate mTORC1 signaling. Previous work indicated that mTORC1 activation by the phorbol ester PMA (phorbol 12-myristate 13-acetate) depends upon PKCs and may involve MEK. However, the precise mechanism(s) through which they activate mTORC1 remains unclear. Recent studies have implicated both the ERKs and the ERK-activated 90-kDa ribosomal S6 kinases (p90(RSK)) in activating mTORC1 signaling via phosphorylation of TSC2 (a regulator of mTORC1) and/or the mTORC1 component raptor. However, the relative importance of each of these kinases and phosphorylation events for the activation of mTORC1 signaling is unknown. The recent availability of MEK (PD184352) and p90(RSK) (BI-D1870) inhibitors of improved specificity allowed us to address the roles of these protein kinases in controlling mTORC1 in a variety of human and rodent cell types. In parallel, we used specific shRNAs against p90(RSK1) and p90(RSK2) to further test their roles in regulating mTORC1 signaling. Our data indicate that p90(RSKs) are dispensable for the activation of mTORC1 signaling by phorbol esters in all cell types tested. Our data also reveal striking diversity in the requirements for MEK/ERK in the control of mTORC1 between different cell types, pointing to additional signaling connections between phorbol esters and mTORC1, which do not involve MEK/ERK. This study provides important information for the design of efficient strategies to combat the hyperactivation of mTORC1 signaling by oncogenic pathways.     

Visfatin promotes angiogenesis by activation of extracellular signal-regulated kinase 1/2

Adipose tissue is highly vascularized and requires the angiogenic properties for its mass growth. Visfatin has been recently characterized as a novel adipokine, which is preferentially produced by adipose tissue. In this study, we report that visfatin potently stimulates in vivo neovascularization in chick chorioallantoic membrane and mouse Matrigel plug. We also demonstrate that visfatin activates migration, invasion, and tube formation in human umbilical vein endothelial cells (HUVECs). Moreover, visfatin evokes activation of the extracellular signal-regulated kinase 1/2 (ERK1/2) in endothelial cells, which is closely linked to angiogenesis. Inhibition of ERK activation markedly decreases visfatin-induced tube formation of HUVECs and visfatin-stimulated endothelial cell sprouting from rat aortic rings. Taken together, these results demonstrate that visfatin promotes angiogenesis via activation of mitogen-activated protein kinase ERK-dependent pathway and suggest that visfatin may play important roles in various pathophysiological angiogenesis including adipose tissue angiogenesis.     

MEK kinase 2 and the adaptor protein Lad regulate extracellular signal-regulated kinase 5 activation by epidermal growth factor via Src

Lad is an SH2 domain-containing adaptor protein that binds MEK kinase 2 (MEKK2), a mitogen-activated protein kinase (MAPK) kinase kinase for the extracellular signal-regulated kinase 5 (ERK5) and JNK pathways. Lad and MEKK2 are in a complex in resting cells. Antisense knockdown of Lad expression and targeted gene disruption of MEKK2 expression results in loss of epidermal growth factor (EGF) and stress stimuli-induced activation of ERK5. Activation of MEKK2 and the ERK5 pathway by EGF and stress stimuli is dependent on Src kinase activity. The Lad-binding motif is encoded within amino acids 228 to 282 in the N terminus of MEKK2, and expression of this motif blocks Lad-MEKK2 interaction, resulting in inhibition of Src-dependent activation of MEKK2 and ERK5. JNK activation by EGF is similarly inhibited by loss of Lad or MEKK2 expression and by blocking the interaction of MEKK2 and Lad. Our studies demonstrate that Src kinase activity is required for ERK5 activation in response to EGF, MEKK2 expression is required for ERK5 activation by Src, Lad and MEKK2 association is required for Src activation of ERK5, and EGF and Src stimulation of ERK5-regulated MEF2-dependent promoter activity requires a functional Lad-MEKK2 signaling complex.     

Differential regulation of IGF-II-induced IL-8 by extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinases in human keratinocytes

In order to study the relationship between insulin like growth factor-II (IGF-II) and interleukin-8 (IL-8) that are upregulated in psoriasis, we monitored IL-8 expression in IGF-II-treated human keratinocytes and explored the signaling pathways of IL-8 expression by IGF-II. IGF-II increased the IL-8 mRNA and protein levels in human keratinocytes. The upregulation of IL-8 expression by IGF-II was reduced by pretreatment with inhibitors of tyrosine kinase, Src, PI3-kinase, and ERK, but not by p38. Furthermore, IGF-II remarkably increased the DNA binding activities of NF-kappaB and AP-1, and the IL-8 promoter activity. However, cotransfection with IkappaB mutant blocked the IGF-II-induced IL-8 promoter activity. In addition, cotransfection with dominant negative MEK1 mutant, but not with dominant negative p38 mutant, blocked the IGF-II-induced IL-8 promoter activity. These results suggest that IGF-II is involved in the pathogenesis of psoriasis by inducing IL-8 gene expression through the tyrosine kinase-Src-ERK1/2-AP-1 pathway, and the PI3-kinase and NF-kappaB pathway.     

NF-kappaB activation by double-stranded-RNA-activated protein kinase (PKR) is mediated through NF-kappaB-inducing kinase and IkappaB kinase

The interferon (IFN)-inducible double-stranded-RNA (dsRNA)-activated serine-threonine protein kinase (PKR) is a major mediator of the antiviral and antiproliferative activities of IFNs. PKR has been implicated in different stress-induced signaling pathways including dsRNA signaling to nuclear factor kappa B (NF-kappaB). The mechanism by which PKR mediates activation of NF-kappaB is unknown. Here we show that in response to poly(rI). poly(rC) (pIC), PKR activates IkappaB kinase (IKK), leading to the degradation of the inhibitors IkappaBalpha and IkappaBbeta and the concomitant release of NF-kappaB. The results of kinetic studies revealed that pIC induced a slow and prolonged activation of IKK, which was preceded by PKR activation. In PKR null cell lines, pIC failed to stimulate IKK activity compared to cells from an isogenic background wild type for PKR in accord with the inability of PKR null cells to induce NF-kappaB in response to pIC. Moreover, PKR was required to establish a sustained response to tumor necrosis factor alpha (TNF-alpha) and to potentiate activation of NF-kappaB by cotreatment with TNF-alpha and IFN-gamma. By coimmunoprecipitation, PKR was shown to be physically associated with the IKK complex. Transient expression of a dominant negative mutant of IKKbeta or the NF-kappaB-inducing kinase (NIK) inhibited pIC-induced gene expression from an NF-kappaB-dependent reporter construct. Taken together, these results demonstrate that PKR-dependent dsRNA induction of NF-kappaB is mediated by NIK and IKK activation.     

Glycogen synthase kinase 3 beta is a natural activator of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 1 (MEKK1)

Glycogen synthase kinase 3beta (GSK3 beta) is implicated in many biological events, including embryonic development, cell differentiation, apoptosis, and insulin response. GSK3 beta has now been shown to induce activation of the mitogen-activated protein kinase kinase kinase MEKK1 and thereby to promote signaling by the stress-activated protein kinase pathway. GSK3 beta-binding protein blocked the activation of MEKK1 by GSK3 beta in human embryonic kidney 293 (HEK293) cells. Furthermore, co-immunoprecipitation analysis revealed a physical association between endogenous GSK3 beta and MEKK1 in HEK293 cells. Overexpression of axin1, a GSK3 beta-regulated scaffolding protein, did not affect the physical interaction between GSK3 beta and MEKK1 in transfected HEK293 cells. Exposure of cells to insulin inhibited the activation of MEKK1 by GSK3 beta, and this inhibitory effect of insulin was abolished by the phosphatidylinositol 3-kinase inhibitor wortmannin. Furthermore, MEKK1 activity under either basal or UV- or tumor necrosis factor alpha-stimulated conditions was reduced in embryonic fibroblasts derived from GSK3 beta knockout mice compared with that in such cells from wild-type mice. Ectopic expression of GSK3 beta increased both basal and tumor necrosis factor alpha-stimulated activities of MEKK1 in GSK3 beta(-/-) cells. Together, these observations suggest that GSK3 beta functions as a natural activator of MEKK1.     

ABIN-2 forms a ternary complex with TPL-2 and NF-kappa B1 p105 and is essential for TPL-2 protein stability

NF-kappa B1 p105 forms a high-affinity, stoichiometric interaction with TPL-2, a MEK kinase essential for TLR4 activation of the ERK mitogen-activated protein kinase cascade in lipopolysaccharide (LPS)-stimulated macrophages. Interaction with p105 is required to maintain TPL-2 metabolic stability and also negatively regulates TPL-2 MEK kinase activity. Here, affinity purification identified A20-binding inhibitor of NF-kappa B 2 (ABIN-2) as a novel p105-associated protein. Cotransfection experiments demonstrated that ABIN-2 could interact with TPL-2 in addition to p105 but preferentially formed a ternary complex with both proteins. Consistently, in unstimulated bone marrow-derived macrophages (BMDMs), a substantial fraction of endogenous ABIN-2 was associated with both p105 and TPL-2. Although the majority of TPL-2 in these cells was complexed with ABIN-2, the pool of TPL-2 which could activate MEK after LPS stimulation was not, and LPS activation of TPL-2 was found to correlate with its release from ABIN-2. Depletion of ABIN-2 by RNA interference dramatically reduced steady-state levels of TPL-2 protein without affecting levels of TPL-2 mRNA or p105 protein. In addition, ABIN-2 increased the half-life of cotransfected TPL-2. Thus, optimal TPL-2 stability in vivo requires interaction with ABIN-2 as well as p105. Together, these data raise the possibility that ABIN-2 functions in the TLR4 signaling pathway which regulates TPL-2 activation.     

The mechanism of dephosphorylation of extracellular signal-regulated kinase 2 by mitogen-activated protein kinase phosphatase 3.

The mitogen-activated protein (MAP) kinase phosphatase-3 (MKP3) is a dual specificity phosphatase that specifically inactivates one subfamily of MAP kinases, the extracellular signal-regulated kinases (ERKs). Inactivation of MAP kinases occurs by dephosphorylation of Thr(P) and Tyr(P) in the TXY kinase activation motif. To gain insight into the mechanism of ERK2 inactivation by MKP3, we have carried out an analysis of the MKP3-catalyzed dephosphorylation of the phosphorylated ERK2. We find that ERK2/pTpY dephosphorylation by MKP3 involves an ordered, distributive mechanism in which MKP3 binds the bisphosphorylated ERK2/pTpY, dephosphorylates Tyr(P) first, dissociates and releases the monophosphorylated ERK2/pT, which is then subjected to dephosphorylation by a second MKP3, yielding the fully dephosphorylated ERK2. The bisphosphorylated ERK2 is a highly specific substrate for MKP3 with a k(cat)/K(m) of 3.8 x 10(6) m(-1) s(-1), which is more than 6 orders of magnitude higher than that for small molecule aryl phosphates and an ERK2-derived phosphopeptide encompassing the pTEpY motif. This strikingly high substrate specificity displayed by MKP3 may result from a combination of high affinity binding interactions between the N-terminal domain of MKP3 and ERK2 and specific ERK2-induced allosteric activation of the MKP3 C-terminal phosphatase domain.     

Phosphorylation of MEK1 by cdk5/p35 down-regulates the mitogen-activated protein kinase pathway.

Cyclin-dependent protein kinase 5 (cdk5), a member of the cdk family, is active mainly in postmitotic cells and plays important roles in neuronal development and migration, neurite outgrowth, and synaptic transmission. In this study we investigated the relationship between cdk5 activity and regulation of the mitogen-activated protein (MAP) kinase pathway. We report that cdk5 phosphorylates the MAP kinase kinase-1 (MEK1) in vivo as well as the Ras-activated MEK1 in vitro. The phosphorylation of MEK1 by cdk5 resulted in inhibition of MEK1 catalytic activity and the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2. In p35 (cdk5 activator) -/- mice, which lack appreciable cdk5 activity, we observed an increase in the phosphorylation of NF-M subunit of neurofilament proteins that correlated with an up-regulation of MEK1 and ERK1/2 activity. The activity of a constitutively active MEK1 with threonine 286 mutated to alanine (within a TPXK cdk5 phosphorylation motif in the proline-rich domain) was not affected by cdk5 phosphorylation, suggesting that Thr286 might be the cdk5/p35 phosphorylation-dependent regulatory site. These findings support the hypothesis that cdk5 and the MAP kinase pathway cross-talk in the regulation of neuronal functions. Moreover, these data and the recent studies of Harada et al. (Harada, T., Morooka, T., Ogawa, S., and Nishida, E. (2001) Nat. Cell Biol. 3, 453-459) have prompted us to propose a model for feedback down-regulation of the MAP kinase signal cascade by cdk5 inactivation of MEK1.